The rapid development in LED efficiency in recent years has resulted in the adoption of LEDs in illumination applications, replacing incandescent bulbs in many applications and beginning to replace cold cathode fluorescent lamps (CCFLs) and fluorescent tube lamps in some specialty lighting applications. Whereas LED applications in the non-illumination sectors have many well established designs for drive circuitry, the same cannot be said for LEDs operated directly from the mains power supply. Unlike other applications, LED lamp products used for illumination have to compete against incandescent lamps and CCFLs with low entry cost. As such, it is not very competitive to use expensive drive circuitry to operate LED lamps off the mains because the replacement cost versus other lamp technologies will be higher and the payback time will be longer.
In non-illumination applications, usually there is a readily available low voltage power supply. This, plus the use of constant current drivers, enables a rather cheap solution to drive LEDs in instrumentation and displays in many end applications. For LEDs to be operated from the mains directly, the first challenge is the cost of the AC to DC power adapter. In the current state of art, a good power adapter for LED operation with a constant current source is almost half the cost of the overall LED lamp. Typically a switched mode power supply is implemented. Other existing LED power supply schemes include half-wave rectification, full wave rectification and rectification with smoothing capacitors and inductors. However, these schemes introduce the problems of lower power factor and high total harmonic distortion to the power supply.
Power adapters with constant current drivers typically use a switch mode power supply together with circuitry to generate a constant current for driving LEDs. While the constant DC current provides efficient LED operation, the drive circuit and power adapter have a high component count and consequently a high cost is involved. In addition, a large amount of space is taken up by this design due to the size and number of components used. This is rather undesirable when the power adapter and drive circuit have to be fitted into the size of a conventional light bulb.
Full wave rectification circuit power adapters and drive schemes have an advantage over the switch mode power supply in that they have fewer components and therefore require less space and are relatively low cost. However, one limitation of this design is that the voltage and current follow half-sinusoidal waveforms, which are not suitable for driving LEDs.
Half-wave rectification is not competitive due to low light output as a result of missing half a cycle of operation in DC mode. While this can be rectified by providing additional LEDs in the circuit to operate in the reverse cycle, the cost of the LEDs is doubled.
A typical LED, such as an InGaN LED, typically has a near zero forward current until the turn-on voltage followed by a steep rise in forward current for a small increase in voltage. In a typical design, the forward voltage is designed to be at the maximum allowable through the LED at peak cycle. The resultant average drive current for the whole cycle is much lower than that of the DC drive current. This is partly due to the high turn-on voltage of the LED and this results in lower flux output for a full wave rectification-based LED module.
The waveform of the current and voltage generated by the full wave rectification circuit is not optimal in light output efficiency in terms of lumens per watt of the system. This is because only a small part of the power cycle is at the maximum allowable drive current and consequently only a small part of the power cycle is at the maximum light output, as shown in FIG. 1. The time-average light output is much lower than in DC mode. Moreover, the LED is constrained in the maximum voltage and maximum drive current it can tolerate without creating electrical overstress. Full wave rectification without modification of the sinusoidal forward voltage and forward current waveforms would severely limit the drive current through the LEDs throughout much of the power cycle, resulting in a rather low overall average DC current and low average power. FIG. 2 shows an example of the luminous intensity (Iv) waveform resulting from a full wave rectified power supply. For about 40% of the duration of the power cycle, the light output is practically zero and for 70% of the duration of the power cycle, the light output is less than half the peak value.
In conventional bridge rectification circuits, a higher amount of heat is generated due to the elevated forward voltage of the rectified power supply over part of the power cycle, which leads to a higher LED junction temperature. The light output of the LED decreases as the junction temperature rises as a result of the LED thermal characteristics. This leads to a second contribution to light output drop, over and above the effect caused by current saturation in the LED junction.
Some full wave rectification circuits include a filter capacitor which acts to smooth out the variation in the DC waveform. However, the capacitive load causes harmonic distortion to the power supply and there is a drop in the power factor of the system. An example of such a circuit used to drive LEDs directly off the AC mains supply is shown in FIG. 3. The bridge circuit converts the sinusoidal waveform from the AC mains to a full-wave rectified DC waveform. The circuit uses a capacitor C1 and an inductor L1 to smooth the resulting DC waveform, while the value of resistance for resistor R1 is chosen to limit the current flowing through the LEDs LED1 to LEDn. Sometimes an over-current protection device D1 is incorporated in the circuit. The main losses of the system include power loss in resistor R2 of the AC-to-DC conversion block and power loss in resistor R1 used to limit the current flowing through the LEDs. In one example of the circuit shown in FIG. 3, six InGaN LEDs with a typical voltage of 3.3V are used in the LED string. In another design, two LED strings, each having six LEDs, are used in parallel. Both of these designs have approximately 20V drop across the LEDs in total with the typical average voltage of 3.3V per LED. The excess of the rectified voltage over the LEDs is taken up by the resistor R1 and dissipated as heat.
U.S. Pat. No. 7,272,018 discloses another prior art power adapter design in the form of a switched mode power supply scheme with power factor correction. The complex circuit has a higher component count and increases the cost of the design to supply DC power to an LED string.
U.S. Pat. No. 6,600,670 discloses another switch mode power supply scheme to provide a constant DC supply, which needs multiple types of components and has a high component count. For a low cost product such as an LED lamp, these schemes are not suitable although the DC power supply quality is good in terms of high power factor and low harmonic distortion to the power supply.
Another problem with driving LEDs directly off the AC mains is the variation in forward voltage of individual LEDs due to their mass production. For example, the operating forward voltage of a white LED typically ranges from about 2.8V to about 3.5V for higher grade LEDs, whilst lower grade LEDs range from about 2.8V up to about 3.9V. When the LEDs are stringed in series, the number of LEDs used needs to be determined precisely. However, due to the variation in the forward voltage from LED to LED, there is a need to specify a narrow forward voltage range for the LEDs. This requirement necessitates tight forward voltage binning for the LEDs. This increases the manufacturing cost of LEDs to cover the cost of rejects in the forward voltage binning process. If no forward voltage binning is performed, there will be a variation of total forward voltage coming from the LEDs, resulting in an increase in light output variation of the LED devices.
As stated above, many circuit designs for powering LEDs from an AC power supply utilise inductive components, such as transformers, inductors or magnetic coils. These components introduce electromagnetic radiation noise and as a result require additional EMI suppression measures in the circuit design. In addition, the magnetic coils cause humming noise in the presence of magnetic parts, for example, in fluorescent tube lighting fixtures.
Further circuit designs for powering LEDs from an AC power supply are disclosed in U.S. Pat. No. 7,344,275, U.S. Pat. No. 7,066,628, U.S. Pat. No. 6,867,575, U.S. Pat. No. 6,830,358, U.S. Pat. No. 6,636,027, U.S. Pat. No. 6,461,019 and U.S. Pat. No. 6,072,280. However, these designs suffer from one or more of the aforementioned problems or drawbacks.
Drawbacks are also encountered with conventional systems for controlling colored LEDs being powered from an AC supply. In conventional LED color control systems, either pulsed width modulation or resistive switches are used to control the color and brightness levels of red, green and blue (RGB) LEDs to produce the color gamut. Pulse width modulation has the advantage of high efficiency in lumens per watt, but a drawback is the need for complex circuitry to implement the color mixing, including the use of LED drivers, color control integrated circuits (ICs), a microprocessor and a power adapter for low voltage supply. Resistive switches utilize series resistance to reduce the current flowing through the LED circuits. Although this method is cheaper and simpler than pulsed width modulation, it reduces the efficiency of the illumination system through heat losses in the series resistance, especially at high dimming levels.